Mark-up languages are languages for defining the content and structure (layout) of electronic documents. Most of the markup languages in use today are versions of the Standard Generalized Mark-up Language (SGML), which is a meta-language (a language for describing languages) standardized by the International Standards Organization (ISO). Examples of mark-up languages include the Hyper-Text Mark-up Language (HTML) which is an electronic publishing language that is used to define pages of information on the World Wide Web, Virtual Reality Mark-up Language (VRML) which is used to deliver three-dimensional images through the Internet, and Extensible Mark-up Language (XML) which is an abbreviated and simplified version of SGML that is designed to make information self-describing, and thus make it easier to define document types and to write programs to handle them.
Mark-up languages are most often used to describe electronic documents to browsers, although other applications can be used as clients. Their use allows the documents to be changed without impacting the browser, and allows the same browser to be used with different Web sites irrespective of server technology. The same type of flexibility is desirable for user interfaces. The desire is to be able to create user interfaces that can be changed or extended, or integrated with user interfaces of different other systems, without changes to the underlying systems.
The Mozilla organization of Netscape Communications Corporation has attempted to do this for graphical interfaces by defining the XML-based User Interface Language (XUL). XUL is an application of XML; it is XML with specific meanings defined for a few element types, and into which HTML and JavaScript can be scattered. XUL is designed to be used to define and develop graphical user interfaces (GUIs) which will work on multiple platforms (cross-platform GUIs). An additional benefit of designing a GUI in XUL is that it may be easily modified or expanded (added-to). However, not all platforms have, or can have, a GUI. A prime example are platforms like voice messaging systems and interactive voice response systems that interact with users via telephones and therefore must utilize a telephony user interface (TUI) made up of DTMF (Touch-Tone) signals and/or spoken instructions. Since XUL is designed for GUIs only, XUL does not provide its benefits to these platforms.
Conversely, voice XML (VXML, XML combined with voice-recognition technology) is being used by a consortium of companies to provide a telephony user interface (TUI) to Web content. However, not all platforms have, or can have, a TUI. A prime example is conventional Web browsers, which are not voice-enabled. Since VXML is designed for TUI only, VXML does not provide its benefits to these platforms.